Alkylene oxides (vicinal epoxy alkanes), and particularly propylene oxide, are very valuable and widely used chemicals. They have been polymerized with a wide variety of monomers to yield polymers which are useful in coating compositions and in the manufacture of molded articles. Alkylene oxides have also been reacted with alcohols to yield monalkyl ethers which have utility as solvents in many commercial processes and which are useful as components for synthetic turboprop and turbojet lubricants.
There are many methods known in the art for the production of alkylene oxides and, most notably, propylene oxide. One of the oldest methods is the so-called "chlorohydrin process" which involves the reaction of chlorine and water to form hypochlorous acid which is then reacted with propylene to form propylene chlorohydrin. The propylene chlorohydrin is then dehydrohalogenated to yield propylene oxide. Another method to obtain propylene oxide is by the liquid phase oxidation of propylene with organic peracids. Still another method involves the liquid phase oxidation of propylene with t-butyl hydroperoxide and/or ethylbenzene hydroperoxide.
The aforementioned known methods have serious disadvantages associated therewith. For example, the "chlorohydrin process" requires the use of chlorine which is relatively expensive and corrosive in nature, requiring special handling and expensive equipment. Additionally, the chlorohydrin saponification to propylene oxide consumes alkali chemicals such as caustic soda or lime, producing a large aqueous waste stream containing chloride salts, which require costly treatment prior to discharge from the plant. The oxidation of propylene with peracids is a potentially dangerous operation and expensive equipment is needed to guard against potentially explosive hazards when working with the peracids. Another disadvantage of this method is the high cost of peracids. The t-butyl hydroperoxide and ethylbenzene hydroperoxide processes have the disadvantages of being capital-intensive, multi-step, rather complicated processes. Furthermore, these processes require co-feedstocks of isobutane or ethylbenzene, thus constraining the practical utility of the processes for propylene oxide manufacture.
Another method which has received considerable attention in the literature is the direct oxidation of hydrocarbons with an oxygen-containing gas. This method suffers from the disadvantage that it is not specific for the production of alkylene oxides but produces a variety of other compounds including acids, esters, ethers, and oxides of carbon including carbon monoxide and carbon dioxide. The reaction does, however, possess two attributes which recommend it highly for commercial utilization, i.e., inexpensiveness of starting materials and simplicity of operation. It is primarily for these reasons that much attention in recent years has been directed to improvements in methods for the production of alkylene oxides from the direct oxidation of hydrocarbons even though the producer must necessarily contend with the concurrent production of a variety of undesired products.
By way of illustration, the prior art methods which attempted to produce propylene oxide by the oxidation of propane such as that disclosed in U.S. Pat. No. 2,530,509, assigned to Linde Air Products Company, were only partially successful. The majority of the prior art methods used conventional vertical columns and differed from each other by variations in lengths and diameter of the column, temperature, pressure, etc. However, all of these methods suffered one common disadvantage--the temperature of the reactants varied throughout the length of the column.
The temperature variations are easily explained since the oxidation reactions are exothermic and the amount of heat evolved differs with each reaction which is taking place. Thus, at various increments along the tube, conditions existed which favored the direction of the oxidation to products other than propylene oxide. These prior art methods necessitated the use of elaborate and expensive cooling apparatus.
Further developments in the art constituted attempts to maximize the desired olefin oxide production while minimizing by-product formation. For example, U.S. Pat. No. 3,132,156, assigned to Union Carbide Corporation, discloses the vapor phase oxidation of saturated aliphatic hydrocarbons to olefin oxides. The method described in this '156 patent is said to provide enhanced olefin oxide production as high as 46.2 lbs per 100 lbs of C.sub.3 consumed which calculates to be about 33 percent (molar) selectivity. While this level os selectivity constituted an improvement, it remains less than might be desired from a commercial standpoint. Likewise, Canadian Pat. No. 968,364, assigned to Union Carbide Corporation, discloses the indirect oxidation of olefins via the oxidation of methanol to a free radical intermediate which in turn, epoxidizes the olefin. However, the indirect oxidation method disclosed in the Canadian '364 patent has the disadvantage of requiring the use of a solvent together with subsequent solvent separation step(s). Accordingly, new methods of producing olefin oxides that combine enhanced selectivity with a simple, inexpensive process would be highly desirable.